The present invention is directed to a Raman detection method and apparatus and, in particular, to one incorporating molecular resonances in surface-enhanced Raman spectroscopy (SERS).
The essential components of a Raman detection system are 1) a source of monochromatic light (e.g., a laser providing a single wavelength/color), which impinges on a scattering object to be chemically analyzed, 2) a dispersive system (often a grating), which separates the different wavelengths present in the scattered light, and 3) a light detector, such as a PMT or CCD. The light detector records the intensity of the dispersed scattered light at different wavelengths. The shifts of the scattered wavelengths with respect to the monochromatic light incident on the scattering object and the intensity of the light at shifted wavelengths form the Raman spectrum (FIG. 1). A Raman spectrum contains fingerprint vibrational information on the scattering object, which is the primary advantage of using Raman scattering for chemical analysis. On the other hand, it is a very weak process, as seen in FIG. 2, and is not well suited to detecting chemical species in low concentration. Therefore, it initially found only limited industrial applications.
Because of fingerprint content of the Raman spectra, efforts have been made to improve the various components of the Raman detection system to enhance the spectral intensity. An example is a confocal Raman microscope, which is a commercial instrument that is widely used in scientific and industrial laboratories.
In addition to improving the efficiency of the conventional Raman system, as outlined above, the detection sensitivity can also be enhanced taking recourse to the physical principles involved in the scattering process. One of these processes is resonance Raman scattering, illustrated in FIG. 3, which can enhance the Raman signals, such as by a million times or more, when the Raman probe wavelength (monochromatic laser wavelength) coincides or nearly coincides with the absorption wavelength of the chemical. This high sensitivity comes at the cost of using different laser wavelengths for different chemicals. For that reason, it becomes a very expensive tool for chemical analysis and, therefore, impractical.
Another phenomenon related to the chemical species adsorbed on rough noble metal surfaces is termed as surface-enhanced Raman scattering (SERS). The Raman signals of a chemical species can be enhanced by several orders of magnitude by this process. A tremendous enhancement of the Raman scattering occurs when the chemical species is adsorbed on metal nanoparticles of about 30-150 nm dimensions. It is possible to detect chemical species at extremely low concentrations and, therefore, a potential wide applicability. One can use low expense visible lasers as monochromatic Raman probes in this case.
It is also known that a single molecule adsorbed on noble metal (e.g., silver, gold) nanoparticles can be detected in a few instances while retaining the fingerprint information. This detection method provides an even greater sensitivity with the fingerprint structural content specific to the chemical species adsorbed on the particle surface. However, this single molecule detection is limited to a very few molecules which tend to absorb in the visible spectrum. Therefore, the resonance Raman phenomenon may also be operative in addition to surface-enhanced Raman. Very few chemicals are the subjects of both phenomena occurring under similar physical and chemical conditions. Most chemicals absorb in ultraviolet and have no color.